March 30, 2015
Murphy V2423 – A774 Chassis
This set was made in December 1972 and acquired from Mikey405’s legendary vintage telly bash event in May this year. It came in a vanload of old tellies brought by fellow collector neil29 . The tellies were unloaded from the van and piled up on Mike’s drive (see pic), then some of them were powered up, including this Murphy.
Initially, the valve heaters glowed but nothing else happened. Replacement PL504 and PY88 valves were fitted, but during this operation the wire from the PL504 top cap broke off and was reattached. The set was powered up again. This time there was some sound, but no picture or EHT and after a few seconds, smoke came off a component attached to the line output transformer, then the sound went off. It was quickly condemned.
These sets were notorious for faulty line output transformers and many other problems. Back in the late 1970s our family TV was a 20″ Murphy A774. I’ve wanted to get hold of another one for nostalgic reasons. I remember all the great programmes I watched on it, but I also remember it was unreliable. Sometimes I missed my favourite show because it had broken down. Very frustrating. My memories of the A774 are bittersweet.
Nonetheless I was keen to take on this rather unloved A774, hoping it could be saved. There were some encouraging signs. First, the line output transformer appeared to be a later replacement, with a stick EHT rectifier instead of a DY802 valve. Secondly, there was some sound from the speaker, albeit briefly. The sound stage gets its power from the LOPT so I thought the LOPT must have a little bit of life left in it.
First of all I downloaded a copy of Television magazine July 1977 from http://www.americanradiohistor…..gazine.htm
This has an article on servicing the A774 chassis including a circuit diagram and common faults. With the help of the schematic, I found the wire to the PL504 top cap had been reattached to the wrong tag on the LOPT. After fixing this, I powered up the set. No more smoke, instead EHT and brightness on the screen for the first time. But the raster looked squashed and didn’t fill the screen. I then tried to apply a signal to the set. At that point, I discovered the tuner didn’t work. It was stuck on UHF channel 31. The channel change buttons had no effect.
Ignoring the tuner fault for the time being, I set my RF modulator to channel 31 then connected my test card generator (actually a digital photo viewer with an image of the test card loaded onto it). The picture was squashed with bad vertical linearity. I tried to adjust the height and v.lin controls but this was the best I could get.
Consult the TV magazine article. It suggests replacing the PCL805 valve and its cathode capacitor 220uf 25v (3C18). Tried that, but no fix. In the end, I found the fault – an “ERIE” capacitor 3C17 0.015uf 1000v DC / 350v AC connected to the PCL805 anode. I used a 0.02uF 630v as that was the nearest value I had. Now I could set the height and linearity correctly.
That’s as far as I’ve got. I’m pleased to have obtained a reasonable picture on the set, but there are more faults. The tuner is still stuck. Another fault – the brightness suddenly increases after the set has been switched on for a couple of minutes. The brightness then remains stable. Any ideas on that one? Also, I can’t see any arcing, but for a short time after switching on, there is a slight hissing sound and a smell of ozone – it stops once the set is warm. I wonder if the LOPT is about to go pop
February 18, 2015
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